1. Field of the Invention
This invention relates to a method of producing a low iron loss grain oriented silicon steel sheet not having its properties degraded by strain relief annealing, and more particularly to an improvement of iron loss value in a grain oriented silicon steel sheet after secondary recrystallization annealing without having its secondary recrystallization annealing, which can be realized by imparting non-uniformity to an oxide layer formed on the surface of the sheet to provide regions acting under different tensions or to provide magnetically different regions on the surface.
2. Related Art Statement
Grain oriented silicon steel sheets are mainly utilized as cores for transformers and other electrical machinery and equipment, and are required to have excellent magnetic properties, particularly a low iron loss (represented by the W.sub.17/50 value).
For this purpose, it is demanded to highly align the &lt;001&gt; orientation of secondary recrystallized grains in the silicon steel sheet into the rolling direction and to reduce impurities and precipitates existent in the steel of the final product as far as possible.
Under the above circumstances, there have been attempted great efforts for improving the properties of grain oriented silicon steel sheets up to the present. As a result, the iron loss value has also improved from year to year. Recently, a W.sub.17/50 value of 1.05 W/kg was obtained in a product having a thickness of 0.30 mm.
However, it is strongly demanded to develop electrical machinery and equipment having less power loss in view of the energy crisis existing since several years ago. In this connection, grain oriented silicon steel sheets having a lower iron loss are demanded as a core material.
As a general means for reducing the iron loss of the grain oriented silicon steel sheet, there are mainly known metallurgical means, such as increasing the Si content, decreasing the product thickness, fining of secondary recrystallized grains, reducing impurity contents, highly aligning secondary recrystallized grains into {110}&lt;001&gt; orientation and the like. These metallurgical means already reach to a limit in view of the existing production process, so that it is very difficult to attain an improvement of the properties exceeding the existing values. If any improvement is realized, the actual effect of improving the iron loss is slight for the effort.
Apart from the above general means, Japanese Patent Application Publication No. 54-23647 proposes a method of fining secondary recrystallized grains by forming secondary recrystallization inhibiting regions on the steel sheet surface. In this method, however, the control of secondary recrystallized grain size is unstable, so that such a method can not be said to be practical.
In addition, Japanese Patent Application Publication No. 58-5968 proposes a technique for reducing the iron loss in which a microstrain is introduced into the surface portion of the steel sheet after the secondary recrystallization by pushing a small ball of the type used in a ballpen to the steel sheet surface to conduct refinement of magnetic domains, and Japanese Patent Application Publication No. 57-2252 proposes a technique for reducing the iron loss in which a laser beam is irradiated at intervals of several mm onto the surface of the final product in a direction perpendicular to the rolling direction to introduce high dislocation density regions into the surface portion of the sheet and conduct refinement of magnetic domains. Further, Japanese Patent laid open No. 57-188810 proposes a technique of reducing the iron loss in which a microstrain is introduced into the surface portion of the steel sheet by discharge working to conduct refinement of magnetic domains.
All of these methods are designed to reduce the iron loss by introducing a micro plastic strain into the surface portion of the base metal in the steel sheet after secondary recrystallization to provide refinement of magnetic domains, and are evenly practical and have an excellent effect of reducing the iron loss. However, the effect obtained by the introduction of plastic strain in these methods is undesirably reduced by strain relief annealing after the punching, shearing work, coiling or the like of the steel sheet or by subsequent heat treatment such as baking of the coating layer or the like.
In Japanese Patent laid open No. 61-73886, there is proposed a technique for reducing the iron loss in which a non-uniform elastic strain is given to the steel sheet surface by locally removing a surface coating through a vibrating body forcedly performing reciprocal movement at a moving quantity of not less than 5.times.10.sup.-6 kg m/s. Even in this technique, however, the effect is largely lost by annealing at a temperature above 600.degree. C.
Moreover, when the introduction of micro plastic strain is carried out after the coating treatment, a reapplication of insulative coating should be carried out for maintaining the insulation property, so that the number of steps of the process significantly increases, resulting in rise of cost.
In order to solve the above drawbacks of the conventional techniques, the formation of deficient portions on forsterite film is proposed in Japanese Patent laid open No. 60-92481.
There are described two methods for the formation of deficient portions in the above publication, one being a method of locally forming no forsterite portion and the other being a method of locally forming the deficient portions after the formation of forsterite. Among them, the method of locally removing forsterite is an actually industrial and useful method because in the method of locally forming no forsterite portion, the process control is difficult due to the use of chemical means or means for obstructing the reaction.
On the other hand, as the means for locally removing forsterite after the secondary recrystallization or forsterite formation, there are disclosed chemical polishing, electrolytic polishing, mechanical method of using a rotational disc-like grindstone or an iron needle under a light pressure, and further an optical method using an output-adjusted laser beam or the like. These methods exhibit an effect to a certain extent, respectively. However, the chemical polishing and electrolytic polishing become considerably high in cost. In the use of the rotational disc-like grindstone, it is difficult to control the position following to the disc height in accordance with the surface properties, so that this is unsuitable for industrial production. Moreover, the optical method using the laser beam or the like becomes high in cost.
On the other hand, the use of an iron needle under light pressure is low in cost, but is difficult to control to remove only forsterite and also removes a part of the surface portion of the base metal together with forsterite. As a result, upheaving of the base metal is caused at both sides of the removed portion or deficient portion to considerably lower the lamination factor and the like. That is, the use of the iron needle is difficult to industrially put into practical use.
As a technique for the refinement of magnetic domains, the formation of grooves in the surface of the silicon steel sheet is disclosed in Japanese Patent Application No. 50-35679, and in Japanese Patent laid open Nos. 59-28525, 59-197520, 61-117218 and 61-117284 and is a well-known technique. Since this technique utilizes a phenomenon of magnetic domain refinement through diamagnetic field in the groove space, however, there are many drawbacks that the magnetic flux density (represented by B.sub.10 value) is largely decreased, and the mechanical properties are degraded and the lamination factor is considerably decreased in accordance with the groove form though the above technique is durable to the strain relief annealing.